1. Field of the Invention
This invention relates to a pulse laser apparatus which makes use of gas as an excitation or ionization medium, and more particularly to a mixture of gas which serves as buffer gas for an excitation or ionization medium and control of a laser of a pulse laser apparatus of the type mentioned.
2. Description of the Prior Art
An excimer laser, a metal vapor laser, a TEACO2 laser, a nitrogen laser and so forth are used as pulse laser apparatus which make use of gas as an excitation or ionization medium. An exemplary one of conventional pulse laser apparatus is disclosed, for example, in Collection of Speech Drafts for the Sixth Annual Conference of the Laser Society. No. 21aB3, 1986 and is shown in sectional view in FIG. 1. The conventional pulse laser apparatus shown in FIG. 1 is constructed as a copper vapor laser apparatus which is a kind of metal vapor laser apparatus. Referring to FIG. 1, reference characters 1a and 1b denote electrodes for causing a discharge, reference numeral 2 denotes a discharge tube, 3 a discharge spacing for exciting copper vapor, 4 copper for generating copper vapor therefrom, 5 copper vapor, 6 an insulator member, reference characters 7a and 7b denote resonance mirrors for causing laser oscillation, 8a and 8b flanges for making up an enclosed spacing, reference character 9 denotes a vacuum layer, 10 an insulator tube, 11 an enclosing tube, 12a a gas inlet port and 12b a gas outlet port.
Subsequently, operation will be described. A pulse voltage is applied between the electrodes 1a and 1b to cause discharging in the discharge spacing 3 in which gas is enclosed. The gas in the discharge spacing 3 is heated by the acceleration energy of ions and electrons are generated by the discharging to evaporate the copper 4. The electrons to which high acceleration energy has been provided by the pulse discharge thus collide with the copper vapor 5, whereupon they give the energy thereof to the copper vapor 5 to excite the copper vapor 5 to a higher excitation level of a laser oscillation line. The insulator member 6 plays the role of intercepting the heat from the discharge spacing 3 in order to maintain the discharge spacing 3 at a predetermined copper vapor concentration. Meanwhile, the vacuum layer 9 plays the same role as the insulator member 6 and particularly intercepts the radiation heat. When the copper vapor 5 excited to the higher excitation level of the oscillation line transits to a lower excitation level, light is generated. The light thus generated is optically amplified by the resonance mirrors 7a and 7b and outputted as laser light to the outside so that it is utilized in various industrial fields such as, for example, for laser working. The gas is supplied from a gas supplying apparatus through the gas inlet port 12a into the discharge spacing 3 and then discharged through the gas outlet port 12b.
The conventional copper vapor laser apparatus is constructed in such a manner as described above, and using a laser medium in the form of a mixture of neon gas serving as buffer gas and copper vapor, the copper vapor laser apparatus effects pulse oscillation by flowing excitation pulse current through the discharge tube as shown in FIG. 2A. Here, if the power to be introduced into the discharge spacing is increased by an increase of the electric input of excitation pulses or by an increase of the frequency of pulses, then the temperature of the gas rises so that charged particles may be produced by an excessively great amount in the discharge spacing. Consequently, as seen from the solid line FIG. 2B, the resistance to discharging decreases quickly for a pulse on period for which a pulse is present, and as seen from the solid line FIG. 2C, the discharge voltage applied to the discharge medium does not increase in proportion to an increase of the electric input. As a result, the number of electrons which have the high energy necessary to excite the copper vapor to the higher excitation level of the oscillation line does not increase in proportion to an increase of the electric input for the pulse on period, and the rate at which the copper vapor can be excited to the higher excitation level does not increase. Accordingly, there is a problem that the gain of the laser does not increase but becomes saturated.
Further, if the electric input to be introduced into the discharge spacing is increased, then the discharge current flowing through the discharge spacing further increases due to a decrease of the resistance to the electric input for a pulse on period as seen from the solid line of FIG. 2D, and consequently, the numbers of ions and electrons remaining in the discharge spacing for a pulse off period subsequent to the pulse on period are increased. As a result, the temperature of the gas for the pulse off period becomes so high that the number of copper vapor atoms at the lower excitation level of the laser oscillation line which directly have a bad influence on oscillation immediately before introduction of an excitation pulse is increased as seen from the solid line of FIG. 2E. As a result, there is a problem that, since the gain of the laser increases in proportion to a difference between the number of copper vapor atoms at the higher excitation level and the number of copper vapor atoms at the lower excitation level, the gain of the laser, is reduced by an increase of the number of copper vapor atoms at the higher excitation level.
Further, if the temperature of the gas for a pulse off period is high, the resistance to the discharge immediately before introduction of an excitation pulse is low as seen from the FIG. 2F. The influence of the reduction of the resistance continues to a next pulse on period, and the discharge resistance upon pulse discharge is decreased and the discharge voltage applied to the resistance of the discharge is decreased. As a result, there is a problem that the number of electrons having energy necessary to excite copper vapor to a higher excitation level of the oscillation line is decreased for the pulse on time, and the rate at which the copper vapor can be excited to a higher excitation level is decreased, and consequently, the gain of the laser is decreased.
The conventional laser apparatus further has problems in stability, deterioration of the quality of a laser beam and so forth such that the laser medium is deteriorated by impurities or the like and the laser output is reduced.